The present invention relates to a lens system suited to forming images using visible light and near-infrared light.
Japanese Laid-Open Patent Publication No. 2003-121737 discloses the provision of a master focus (rear focus)-type four-group zoom lens with a bright f number, a high zoom ratio, and high optical performance across the entire zoom range. The zoom lens disclosed in this Publication is disclosed as being composed, in order from the object side, of a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, and a fourth lens group with positive refractive power, with the first and third lens groups being fixed. The third lens group is composed of a single positive lens and has at least one aspherical surface. The second lens group is moved in one direction to change the magnification (i.e., “zoom”), and the fourth lens group is moved non-linearly to correct movement of the image plane that accompanies zooming and is moved toward the subject to shift the focus from a subject at infinity to a subject at close range. The publication also states that the fourth group is composed of two positive lenses and one negative lens, and that the zoom lens is composed of a total of ten lenses.
Japanese Laid-Open Patent Publication No. 2006-3589 discloses a varifocal lens that has a high zoom ratio and is capable of achieving high optical performance across the entire zoom range for light of the visible range to the near-infrared range. In the lens disclosed in this Publication, a positive first group, a negative second group, a positive third group, and a positive fourth group are disposed in that order from the object side. The publication additionally states that when zooming from the wide-angle end to the telephoto end, the first group and the third group are fixed on the optical axis, the second group is moved along the optical axis toward the image to change the magnification, and the fourth group is independently moved along the optical axis to correct movement of the image plane that accompanies the zooming. In order from the object side, the first group is composed of a cemented lens composed of negative and positive lenses and a positive lens, the second group is composed of a negative lens and a cemented lens composed of negative and positive lenses, the third group is composed of a cemented lens composed of positive and negative lenses, and the fourth group is composed of a positive lens, a negative lens and a positive lens, where at least one surface is aspherical.
A telephoto type lens system with four groups in a positive, negative, positive, and positive arrangement is often used as a lens system for monitoring applications. Due to the increasingly high performance of image pickup elements in recent years, higher resolution is now demanded for such lens systems for monitoring applications. To achieve higher resolution, favorable correction of axial chromatic aberration and chromatic aberration of magnification is important. Telephoto lenses that are important for monitoring purposes tend to have high axial chromatic aberration compared to wide-angle lenses. To realize day and night operation, it is also necessary to thoroughly suppress axial chromatic aberration for the range of visible wavelengths and near-infra red wavelengths, for example, up to a wavelength range of 800 to 900 nm. On the other hand, it is necessary to lower the F number to achieve bright images, which makes it difficult to increase the depth of focus.
In monitoring applications, there is demand for a lens system that does not have an autofocus mechanism or a motorized focusing mechanism and also does not have a motorized zoom mechanism. There is also demand for such a lens system to be bright and to have little axial chromatic aberration.